BJT in Saturation Mode
Section 4.5
Schedule
9
2/11
Tuesday
Physics of a BJT
L
2/11
Tuesday
Measure Beta of a transistor
10
2/13
Thursday PNP
4.3, 4.54.6
11
2/18
Tuesday
4.5
L
2/18
Tuesday
12
2/20
BJT in saturation
BJT in saturation/BJT implementation of an
NAND gate
Thursday Small signal model [homework: small eq.
circuit]
4.1-4.3
Outline
•
•
•
•
Modes of Operations
Review of BJT in the active Region
BJT in Saturation Mode
Digital Integrated Circuits
Modes of Operation
BE\BC
Forward Biased
Reverse Biased
Forward Biased
Saturation
Active
Reverse Biased
Reverse Active
Mode
Cut-Off
Applications:
1. Saturation and cut-off mode are used in digital circuits.
2. Active mode is used in the amplifier design.
Voltage and Current Polarities of
NPN and PNP transistors
A “fat” voltage
between collector
and emitter voltage
places a transistor in
the active region!
A “skinny” voltage between collector
and emitter voltage places a transistor
in the active region!
Review
Review
• PN Junction
– Reverse Bias
– Forward Bias
– BJT in the Active Mode
Review: Forward Bias Diode
Depletion region shrinks due to charges from the battery.
The electric field is weaker.
Majority carrier can cross via diffusion;
Greater diffusion current.
Current flows from P side to N side
Review: PN Junction under
Reverse Bias
Reverse: Connect
the + terminal to the
n side.
Depletion region widens.
Therefore, stronger E.
E
Minority carriers cross
the PN junction easily
through diffusion.
Current is composed
mostly of drift current contributed
by minority carriers.
np to the left and pn to the right.
Current from n side to p side,
the current is negative.
Operation of an NPN
Transistor in the Active Region
Electrons are injected
into the B; holes to the E.
Electrons
are injected
into the BC junction
Electrons
are swept across
the reversed biased BC
Thin Base Region
The base region is made thin in order to reduce recombination
as electrons travel from BE junction to BC junction.
Highly Doped Emitter
In order to emphasize the current contribution
due to the electrons (which can cross the BC junction),
the emitter is heavily doped by N type materials.
Base Current
The proportional of hole current and electron current
is determined by dopants (ND and NA).
Even though the presence of holes are minimized, a small
number holes still must enter through the base.
Electrons in the Base
Electrons injected into
the base; high electron
density at x1.
Electrons are swept
Into the collector;
low electron density at x2
The electron gradient allows electrons to travel through diffusion.
Recombination
Recombination
Base must supply holes that will enter the emitter and
for recombination with the electrons.
Extension of a PNP transistor
(NPN transistor)
(PNP transistor)
1. Base-emitter junction is forward
biased.
2. Holes are injected into the base.
3. Base-emitter junction is reverse
Biased.
4. Injected holes in the base is swept
across the base-collector junction by
the electric field.
BJT Current
Assumption:
BEJ: Forward Biased
BCJ: Reverse Biased
Large Signal Model of a BJT
Called “large” signal model
because this model is
applicable even if VBE
changes from 300 mV to 800 mV
Large-Signal Model of BJT
Transistors
(NPN)
C
E
(PNP)
C
E
Experiments
Saturation Mode
BJT in Saturation Mode
Key assumption so far:
BE=Forward Biased
BC=Reverse Biased
What happens when these assumptions are not true?
Review: Forward Bias Diode
E
Depletion region shrinks due to charges from the battery.
The electric field is weaker.
Majority carrier can cross the junction via diffusion;
Greater diffusion current.
Current flows from P side to N side
Hole Current into the Collector
A reverse biased BCJ keeps
holes in the base.
But as BCJ becomes forward
biased, the strong electric field
which opposes of the movement
of holes into the collector is weakened.
There is now a hole current into the collector.
Net Result: heavy saturation leads to a sharp rise in the base current and a rapid
fall in β.
A Large Signal Model of the BJT
The net collector current decreases as the collector
enter into saturation
General Rules
• As a rule of thumb, we permit soft saturation with
VBC <400 mV because the current in the B-C
junction is negligible, provided that various
tolerances in the component values do not drive
the device into deep saturation.
• For a device in soft saturation or active region, we
approximate IC as Isexp(VBE/VT)
• In the deep saturation region, the collector-emitter
voltage approaches a constant value called VCE, SAT
(about 200 mV).
Voltage and Current Polarities of
NPN and PNP transistors
A “fat” voltage
between collector
and emitter voltage
places a transistor in
the active region!
A “skinny” voltage between collector
and emitter voltage places a transistor
in the active region!
Use 2n3904 npn BJT in
Simulation
(Error!, put 2n3904 here!)
Include 2n3904 (NPN) model
A NAND Gate Implemented
With NPN Transistors
Optional Slides
BJT Inverter
(Define the input voltage as a variable)
Run Parametric Analysis
Parametric Analysis
Select a Wire to Plot
Use Calculator to Plot
Plot with Calculator (Under
Tools)
RTL (Resistor-Transistor Logic)
Vout
VA
VB
First introduced in 1962! (50 years ago!)
What is the logic function?
RTL Based NOR
A
B
Vout
3.6 V
3.6 V
34.05 mV
3.6 V
0V
42.59 mV
0V
3.6 V
42.59 mV
0V
0V
3.6 V
NOR is an universal gate!
If you can build a NOR, you can build any logic.
Diode-Transistor Logic
What is the logic function?
This resistor allow charges
to be drained from the base
Sweep VB
VS: the input voltage at which the output is approximately 2V.
VS~2V
Condition: VA=4V, VC=4V. VB is swept from 0 to 4V
Diode-Transistor Logic
This resistor allow charges
to be drained from the base
Sweep VB
Fixed VA=4V
VCC=4V
Sweep VB from 0 to 4 V
Increase the VS by about one diode drop.
Basic TTL Gate
Diode is replaced by TTL
A “relative “ of 7400LS Gate
Sweep VB
Fixed VA=4V
VCC=4V
Sweep VB from 0 to 4 V
7400 NAND Gate
7400 Schematic
We will revisit this schematic in a couple of weeks!